Medium-earth-altitude satellite based cellular telecommunications

Abstract: A satellite-based cellular telecommunications system employing a constellation of telecommunications satellites in medium earth orbit to provide multibeam radio frequency (rf) communications links for worldwide cellular telephone service with a minimum number of satellites. The telecommunications satellites are placed in a plurality of inclined orbits about the earth at an altitude of between approximately 5600 and 10,000 nautical miles. The characteristics of the orbits, such as the number of orbits, the inclination of each orbit, the number of satellites in each orbit and the altitude of the satellites, are tailored to maximize the coverage area of the satellites and their related line-of-sight elevation angles, while minimizing propagation time delays, the number of beam-to-beam and satellite-to-satellites handovers, and the total number of satellites. The present invention also includes several additional features which essentially eliminate beam-to-beam and satellite-to-satellite handovers, thus dramatically reducing the likelihood of dropouts.

Description: BACKGROUND OF THE INVENTION This invention relates generally to cellular telecommunications and, more particularly, to methods attendant to satellite-based cellular telecommunications systems. The use of mobile cellular telephones has proliferated in the many large metropolitan areas in which cellular telecommunications systems have been installed. Cellular telecommunications systems provide voice communications between a mobile telephone user and fixed telephone users or to other mobile telephone users, as well as data communications for mobile facsimile machines and modem-equipped computers. A typical cellular telephone system includes a grid of service zones or cells, with each cell having a base station situated near its center. A mobile telephone user located in a particular cell is connected to that cell's base station through low-power radio frequency (rf) transmissions. Each base station is connected by trunk lines to a gateway station, which is connected by trunk lines to a local and long distance telephone network. The cellular telecommunications systems in use today are generally land-based systems that have been installed in large metropolitan areas. Small towns and rural areas cannot economically justify the installation and operation of one of these relatively costly systems. To provide cellular telephone service for these areas, satellite-based cellular telecommunications systems have been proposed which would provide world-wide cellular telephone service. These proposed cellular telephone systems typically include a large constellation of telecommunications satellites in low earth orbit at an altitude of between approximately 741 km and 1852 km (400 and 1000 nautical miles), which is just below the Van Allen radiation belt. At these low altitudes, as many as 50 to 80 satellites are required to provide adequate coverage of the entire earth. This results in an extremely costly and complex system with a large number of rapidly changing crosslinks and a large number of beam-to-beam and satellite-to-satellite handovers. Accordingly, there has been a need for a less costly and complex satellite-based cellular telecommunications system. The present invention clearly fulfills this need. AIAA 92-2060 "Odyssey, A Constellation For Personal Communications" - Roger J. Rusch, Peter Cress, Michael Horstein, Robert Huang, Eric Wiswell, March 1992, describes a satellite based cellular telephone system for Continental United States comprising 12 satellites in 10354 km circular orbits with 4 satellites in each of 3 orbit planes inclined at 55°. This constellation guarantees continuous global dual-satellite visibility. Moreover, an initial, reduced constellation of 9 satellites can provide continuous global single satellite visibility. Each satellite is provided with antennas that are designed to provide coverage to only a portion of the total area visible to the satellite. During the period that a satellite will be assigned to a particular region, the satellite attitude will be controlled so that the antennas remain pointed in the desired direction. According to this document, capacity of the system could be expanded by increasing the number of beams or by increasing the number of satellites. Application of TRW, Inc. for authority to construct a new communications satellite system, May 31, 1991, describes a satellite based communications system. In this document, a 12 satellite constellation is used to provide double coverage. A reduced constellation of 9 satellites is also described. According to this document, in the event of a satellite failure, the remaining satellites may be rephased to maximize coverage. The satellites are oriented by program track to position the total pattern boresight to the nominal center of Continental United States with the appropriate offsets to ensure that all of Continental United States is covered. Each ground station is always in contact with two satellites and preparing to transfer operations to a third. The Continental United States ground station provides each satellite with antenna pointing data and monitors the pointing process via telemetry and signal strength measurements. The user antennas are pointed to service area on the earth by executing satellite attitude commands as the satellite passes over the service area. These commands are generated on the ground and stored on board the satellite prior to passover. EP-A-0 331 567 discloses at least two satellites, which are on geosynchronous orbits. This document discloses a method for controlling the communication between mobile phones and one single telecommunication satellite in a specific triangular region of the earth. Each satellite has antennas being focused onto a single ground station located in the center of the single coverage area during the entire period of the time the satellite is active over the coverage area. While the satellite is not active over the coverage area, there is no other specific direction or maintenance of the focus direction onto the central control station. EP-A-0 510 789 discloses a cellular telephone satellite system containing a satellite constellation formed of a plurality of telecommunications satellites to provide an RF communication link with a mobile cellular and fixed telephone stations within a geographical region. At all times at least one of such satellites is visible from the predetermined region at an elevation of no less than 10°. The RF propagation delay between such visible satellite and the cellular station is no greater than 60 msec. The satellite's orbits do not extend in altitude above the earth beyond altitude referred to as medium earth orbit. SUMMARY OF THE INVENTION The present invention resides in the method defined in claim 1. In preferred embodiments the present invention includes features which essentially eliminate beam-to-beam and satellite-to-satellite handovers, thus dramatically reducing the likelihood of dropouts. One of these important features is an assignment of each satellite to a sequence of fixed service regions (rather than a continuously varying coverage area) during each satellite's orbit. The boresight of the satellite's antennas remains centered on a given assigned service region during the entire time the service region is visible to the satellite. This results in an essentially fixed beam pattern thus practically eliminating beam-to-beam handovers. The present invention provides an original satellite-based cellular telecommunications system which, is capable of evolving in phases to a full baseline satellite-based cellular telecommunications system as the traffic demand and revenue stream increase in magnitude. The original system utilizes a minimum number of satellites which can nevertheless serve a substantial portion of the potential subscriber population. Accordingly, the original system of the present invention will reduce the initial capital investment required, while still enabling commercially significant satellite coverage to be achieved in a way which will not adversely affect the anticipated growth toward a full baseline satellite-based cellular telecommunications system. However, it should be appreciated that the original system also provides a complete satellite-based cellular telecommunications system in terms of its coverage regions. In one embodiment of the original system according to the present invention, a six-satellite medium orbit constellation is employed with two satellites in each of three orbital planes. The use of three orbital planes also facilitates the capability to launch the satellites in pairs, which will minimize launch costs and reduce the sequence of launches requires before the original system is operational. However, the original system may also employ a six-satellite medium orbit constellation with each of the satellites residing in their own individual orbital planes. In any event, continuous satellite coverage is provided in a restricted, but economically significant, range of latitudes. Satellite coverage outside of this latitude range has several, typically small gaps during the course of a day, which would result in a small reduction of the time that satellite coverage would be available. In addition to the reduced number of satellites in the original system according to the present invention, an enlarged field-of-view (FOV) is provided for each of the satellites in comparison with the FOV for the satellites in the earlier embodiments. The enlarged FOV is needed due to the fact that fewer satellites will be employed in the original system than will be employed in the full baseline satellite-based telecommunications system. While this has the effect of increasing the coverage area responsibility of each satellite, there will also be a slight reduction in the communication margins in the user-to-satellite direction. Nevertheless, the enlarged FOV (e.g., 38° to 40°) will still be substantially less than the total satellite visibility (e.g., 44°). In order to compensate for this difference in FOV, the original system also employs the directed coverage method according to the present invention. This method of controlling a constellation of satellites utilizes the ability to programmably steer the boresight of the satellite communication antennas to the desired regions of interest. It will be appreciated from the foregoing that the present invention represents a significant advance in the field of satellite-based cellular telecommunications systems. Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of a satellite-based cellular telecommunications system in accordance with the present invention: Figure 2 (not within the scope of the claimed invention) is a schematic illustration of a constellation of telecommunication satellites providing single global land mass coverage of the earth; Figure 3 (not within the scope of the claimed invention) is a schematic illustration of a constellation of telecommunication satellites providing double global land mass coverage of the earth; Figure 4 is a schematic illustration of a constellation of telecommunication satellites providing single hemispheric coverage of the earth; Figure 5 is a schematic illustration of a constellation of telecommunication satellites providing double hemispheric coverage of the earth; Figure 6 is a schematic illustration of a original constellation of telecommunication satellites which provides partial coverage of the earth; Figure 7 is a schematic illustration of a follow-on constellation of telecommunication satellites not covered by the claims and not according to the invention which provides further coverage of the earth; Figure 8 is a schematic illustration of a full baseline constellation of telecommunication satellites not covered by the claims and not according to the invention which provides complete land mass coverage of the earth; Figure 9 is a cartographic illustration of satellite visibility using the original constellation of Figure 6; Figure 10 is a graphical illustration of satellite coverage using the original constellation of Figure 6; Figure 11 is another graphical illustration of satellite coverage using the original constellation of Figure 6; Figures 12A-12E provide a cartographic illustration of satellite coverage using the original constellation of Figure 6; Figures 13A-13G provide a cartographic illustration of the variable antenna pattern using the beam-steering method according to the present invention; Figure 14 is a cartographic illustration of satellite visibility using an alternate constellation wherein each of the satellites reside in their own individual orbital planes; Figure 15 is a graphical illustration of satellite coverage using the original constellation referred to in Figure 14; Figure 16 is another graphical illustration of satellite coverage using the original constellation referred to in Figure 14; and Figure 17 is a flow chart which further illustrates the coordinated boresight steering method according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in the drawings for purposes of illustration, the present invention is embodied in a satellite-based cellular telecommunications system employing a constellation of telecommunications satellites in medium earth orbit to provide multibeam radio frequency (rf) communications links for world-wide land mass cellular telephone service with a minimum number of satellites. The telecommunications satellites are placed in a plurality of inclined orbits about the earth at an altitude of between approximately 10360 km and 18520 km (5600 and 10,000 nautical miles). The characteristics of the orbits, such as the number of orbits, the inclination of each orbit, the number of satellites in each orbit and the altitude of the satellites, are tailored to maximize the coverage area of the satellites and their related line-of-sight elevation angles, while minimizing propagation time delays, the number of beam-to-beam and satellite-to-satellite handovers, and the total number of satellites. The present invention also includes several additional features which essentially eliminate beam-to-beam and satellite-to-satellite handovers, thus dramatically reducing the likelihood of dropouts. Figure 1 illustrates a satellite-based cellular telecommunications system in accordance with the present invention. The cellular telecommunications system includes a constellation of telecommunications satellites 10 in medium earth orbit, a gateway station 12 situated in each region serviced by the satellites and a plurality of mobile cellular telephones 14 located in the service region of the satellite 10. The mobile cellular telephones 14 can also include mobile facsimile machines and modem-equipped computers. The telecommunications satellite 10 provides a multibeam rf communications link with the mobile cellular telephones 14 using two multibeam antennas 16, 18 and an rf communications link with the gateway station 12 using two antennas 20, 22. Multibeam antenna 16 receives rf signals from the mobile cellular telephones 14 on frequency band F 1 . The satellite 10 then amplifies, translates and transmits the rf signals down to the gateway station 12 on frequency band F 2 using antenna 20. The gateway station 12 is connected by trunk lines to a local and long distance telephone network 24, which provides connections to a fixed telephone 26. RF signals are transmitted from the gateway station 12 back to the telecommunications satellite 10 on frequency band F 3 and received using antenna 22. The satellite 10 then amplifies, translates and transmits the rf signals back down to the mobile cellular telephones 14 on frequency band F 4 using multibeam antenna 18. Various types of connections are possible between the different users of the cellular telecommunications system of the present invention. A mobile telephone user can communicate with another mobile telephone user in the same service region of the satellite 10 or in the service region of another satellite 10, or with a fixed telephone user located anywhere in the world. If in communication with another mobile telephone user in the same service region, the rf signals are transmitted up from the mobile cellular telephone 14 to the satellite 10, then down to the gateway station 12, then back up to the satellite 10, and then down to the other mobile cellular telephone 14, and vice versa. This type of connection requires a double hop, which doubles the propagation time delay. If in communication with another mobile telephone user in a different service region, the rf signals are transmitted up from the mobile cellular telephone 14 to the satellite 10, then down to the gateway station 12, then through the local and long distance network 24, as necessary, to the gateway station 12 responsible for the other service region, then up to the satellite 10 responsible for the other service region, and then down to the other mobile cellular telephone 14, and vice versa. This type of connection also requires a double hop. If in communication with a fixed telephone user, the rf signals are transmitted up from the mobile cellular telephone 14 to the satellite 10, then down to the gateway station 12, and then through the local and long distance network 24, as necessary, to the fixed telephone 26, and vice versa. This type of connection required only a single hop. In an alternative embodiment of the present invention, the satellite 10 performs the function of the gateway station 12 for communications between mobile telephone users located in the same service region, thus requiring only a single hop. In this type of connection, the rf signals are transmitted up from the mobile cellular telephone 14 to the satellite 10, the satellite 10 then performs signal processing on the rf signals to provide any necessary switching between the multiple beams, and then the rf signals are transmitted back down to the other mobile cellular telephone 14, and vice versa. The propagation time delay depends on the altitude of the telecommunications satellite 10 and the location of the mobile cellular telephones 14 in the service region. In the present invention, at an altitude of between 10360 km and 18520 km (5600 and 10,000 nautical miles), the time delay from the satellite 10 to the ground and vice versa is limited to 60 msec. Therefore, the single hop time delay is limited to 120 msec. At an altitude of 10360 km (5600 nautical miles), the one hop time delay between two mobile cellular telephones 14 located directly below the satellite 10 is 69 msec., while the one hop time delay between two mobile cellular telephones 14 spaced apart at the minimum elevation angle of 10° is 96 msec. The relationship between station to station propagation time and satellite altitude for a single hop from a station on the ground up to the satellite and back down to the station at maximum range, which occurs when the satellite is at a minimum elevation of 10°, may be shown as follows: The mobile cellular telephones 14 and the gateway station 12 are within view of the satellite 10 at all times at an elevation angle of at least 10°. As the satellite 10 moves out of view of the mobile cellular telephones 14 in a particular service region, another satellite 10 from a constellation comes into view. Each satellite 10 has a relatively narrow antenna footprint that is considerably smaller than the total area of visibility of the satellite. This is because it is not practical to serve the entire area of visibility due to power and complexity constraints. The service region actually covered by the satellite 10 is determined by the composite coverage of the multibeam antennas 16, 18 has 19 beams for a composite footprint coverage of about 23°. Another ring of feeds can be added to the multibeam antenna to provide 37 beams for a composite footprint coverage of about 35°. Each beam has field of view (FOV) of about 5° and is capable of operating with 1/2 W mobile cellular telephone 14 have an omnidirectional antenna. The present invention also includes several additional features which essentially eliminate beam-to-beam and satellite-to-satellite handovers, thus dramatically reducing the likelihood of dropouts. One embodiment features an assignment of each satellite to a sequence of fixed service regions during each satellite's orbit. The boresight of the satellite's antennas 16, 18 remains centered on the assigned service region is visible to the satellite with respect to antennas 20, 22 they are gimballed to point to the appropriate gateway station. The resulting nearly-fixed antenna beam pattern practically eliminates beam-to-beam handovers. The antenna boresight remains centered on the service region either by adjusting the attitude of the spacecraft 10 or by rotating gimballed antennas. Provision may be made for overlap in the coverage of each service region by two successive satellites during the transition in service from one satellite to another. All calls that are placed during the overlap interval are assigned to the arriving satellite. Therefore, only calls placed prior to the overlap interval and extending beyond the overlap interval are subjected to satellite-to-satellite handovers when the departing satellite is reoriented to its new service region. In several embodiments of the present invention, the constellation of telecommunications satellites 10 has three inclined orbital planes which are evenly spaced about the earth with ascending nodes at 120° intervals about the equator. The satellites 10 are placed in medium earth orbit at an altitude of between 10360 km and 18520 km (5600 and 10,000 nautical miles). This range of altitudes is above the Van Allen radiation belt, but substantially below the altitude for geosynchronous orbit. This provides good visibility with a minimum number of satellites without excessive power requirements. At least one of the telecommunications satellites 10 is visible at all times from each mobile cellular telephone 14 and gateway station 12 at an elevation angle of at least 10° to prevent shadowing. Figure 2 shows a constellation of nine satellites which provides global land mass coverage of the earth by at least one satellite 10 at all times with a minimum elevation angle of 10° and thus is not within the scope of the claimed invention. The nine satellites are arranged in three circular orbital planes P 1 , P 2 , P 3 , with three satellites 30a-30c located in orbital plane P 1 , three satellites 32a-32c located in orbital plane P 2 , and three satellites 34a-34c located in orbital plane P 3 . The satellites are evenly spaced at 120° intervals in each orbital plane at an altitude of 5600 nautical miles. Each orbital plane is inclined at an angle of 55° relative to the equatorial plane. The relative phase angles between the satellites in different orbital planes is 80°. Therefore, before one satellite in the constellation disappears from a user's view, at least one additional satellite comes into view above an elevation angle of 10°. Figure 3 shows a constellation of twelve satellites which provides global land mass coverage of the earth by at least two satellites at all times with a minimum elevation angle of 10° and thus also is not within the scope of the claimed invention. The twelve satellites are arranged in the three circular orbital planes P 1 , P 2 , P 3 , with four satellites 30a-30d located in orbital plane P 1 , four satellites 32a-32d located in orbital plane P 2 and four satellites 34a-34d located in orbital plane P 3 . The satellites are evenly spaced at 90° intervals in each orbital plane at an altitude of 10360 km (5600 nautical miles). Each orbital plane is inclined at an angle of 55° relative to the equatorial plane. The relative phase angles between satellites in different orbital planes is 90°. A constellation of telecommunications satellites in circular orbits is preferred for providing global coverage, but a constellation of telecommunications satellites in elliptical orbits is preferred for coverage of limited areas, such as one hemisphere. Figure 4 shows a constellation of six satellites which provides hemispheric coverage of the earth by at least one satellite at all times with a minimum elevation angle of 10°. The six satellites are arranged in three elliptical orbital planes P4, P5, P6, with two satellites 30a-30b located in orbital plane P4, two satellites 32a-32b located in orbital plane P5 and two satellites 34a-34b located in orbital plane P6. Each orbital plane is defined by an apogee altitude of 11668 km (6300 nautical miles), a perigee altitude of 1111 km (600 nautical miles), an inclination angle of 63.4°, an ascending node spacing of 120°, an argument of perigee of 270°, and a relative phase angle between satellites in different orbits of 180°. By placing the apogee at the northernmost latitude reached by the satellite, which is also equal to the inclination angle, the coverage period for the northern latitude is maximized. Figure 5 shows a constellation of nine satellites which provides hemispheric coverage of the earth by at least two satellites at all times with a minimum elevation angle of 10°. The nine satellites are arranged in three elliptical orbital planes P 4 , P 5 , P 6 , with three satellites 30a-30c located in orbital plane P 4 , three satellites 32a-32c located in orbital plane P 5 and three satellites 34a-34c located in orbital plant P 6 . Each orbital plane is defined by an apogee altitude of 11668 km (6300 nautical miles), a perigee altitude of 1111 km (600 nautical miles), an inclination angle of 63.4°, an ascending node spacing of 120°, an argument of perigee of 270°, and a relative phase angle between satellites in different orbits of 180°. Coverage Global Hemispheric Single Double Single Double Number of Satellites 9 12 6 9 Number Orbital Planes 3 3 3 3 Satellites per Plane 3 4 2 3 Apogee Altitude km (nm) 10 360 (5600) 10 360 (5600) 11 668 (6300) 11 668 (6300) Perigee Altitude km (nm) 10 360 (5600) 10 360 (5600) 1 111 (600) 1 111 (600) Inclination Angle 55° 55° 63.4° 63.4° Ascending Node Spacing 120° 120° 120° 120° Argument of Perigee 0° 0° 270° 270° Relative Phase Angle 80° 90° 180° 180° Minimum Elevation Angle 10° 10° 10° 10° The constellations providing single satellite coverage can be initially formed and then the coverage expanded later to provide double coverage by the addition of only three satellites. Conversely, the constellations providing double coverage allow for the failure of one satellite in each orbital plane without the loss of full service. The position of the remaining satellites in the orbital plane can be adjusted to provide single coverage until the satellite is repaired or replaced. Figure 6 shows an original constellation of six satellites which provides continuous, 24-hour satellite visibility (above 10° elevation angle) to all users located in the latitude bands ±(20° to 45°). The six satellites are arranged in three circular orbital planes P 7 , P 8 , P 9 , with two satellites 40a-40b located in orbital plane P 7 , two satellites 42a-42b located in orbital plane P 8 , and two satellites 44a-44b located in orbital plane P 9 . The satellites are evenly spaced at 180° intervals in each orbital plane at an altitude of 10360 km (5600 nautical miles). Each orbital plane is inclined at an angle between 50° and 55° relative to the equatorial plane, with a 53° inclination being the most preferred. The relative phase angles between the satellites in different orbital planes is 0°. Figure 7 shows the growth of the original constellation to a follow-on constellation of nine satellites. As shown in Figure 7, three new satellites have been added to the constellation, such that each of the orbital planes P 7 , P 8 , P 9 include one additional satellite. In other words, orbital plane P 7 includes satellites 40a-40c, orbital plane P 8 includes satellites 42a-42c, and orbital plane P 9 includes satellites 44a-44c. The satellites in each of these orbitals planes are preferably evenly spaced at 120° intervals. Thus, it should be appreciated that at least some of the satellites from the original configuration will need to be relocated from the original 180° interval. This concept of continuing to launch additional satellites beyond the original constellation in order to provide greater coverage leads to the full baseline satellite constellation of Figure 8. A full baseline satellite constellation is one in which complete land mass (including coastal area) coverage can be achieved. As shown in Figure 8, the full baseline satellite-based telecommunications system will preferably include twelve satellites. Accordingly, it should be appreciated that only six satellite launches will be required if the dual launch method is employed. In the full satellite constellation of Figure 8, each of the orbital planes P 7 , P 8 , P 9 include four satellites which are equally spaced at 90° intervals. Thus, for example, orbital plane P 7 is shown to include satellites 40a-40d. In both the follow-on constellation and the full constellation, the preferred inclination angle of the satellites will remain unchanged at 53° relative to the equatorial plane. Similarly, the orbital altitude of the satellites in all cases will preferably remain at 10360 km (5600 nautical miles). However, in the follow-on constellation, the relative phase angle between satellites in different orbital planes is preferably 0°, while the relative phase angle for the full baseline satellite constellation is preferably 30° (e.g., a 12/3/1 satellite constellation). With these configurations, the follow-on constellation will provide continuous 24-hour satellite visibility in the latitude range between 10° and 70° (at a minimum elevation angle of 10°), while the full baseline constellation will provide continuous 24-hour satellite visibility in the latitude range between 0° and 90° (at a minimum elevation angle of 10°). In accordance with the present invention, it is important to understand that there is a significant difference between satellite visibility and satellite coverage or availability. This is because the satellites according to the present invention are capable of employing an antenna FOV which is less than the FOV for the satellite itself. For example, at the preferred altitude of 10360 km (5600 nautical miles) the FOV for each of the satellites will be 44°. However, the satellites 40a-40d, 42a-42d and 44a-44d need only employ multibeam antennas that provide a FOV in a 38° to 40° range. While this difference in degrees may seem relatively small, it actually has a significant effect on the size of ground area that can be covered without the coordinated boresight steering method according to the present invention. Figures 9-11 illustrate satellite visibility for the original constellation of Figure 6. In this regard, Figure 9 shows a map of the earth in which the time periods of satellite visibility are superimposed for the original constellation. The shaded areas illustrate the latitude bands where continuous 24-hour satellite visibility is provided. Thus, for example, the upper shaded area 46 shows that the six satellites of the original constellation will provide continuous visibility across almost all of the United States and Southern Europe. Similarly, the column labeled Hours Per Day shows the number of hours in each day that at least one of the original constellation satellites will be seen from the ground. Figure 10 shows the percent of time that satellite visibility can be provided according to the present invention across all latitude bands. Thus, for example, at a minimum elevation angle of 10°, it should be understood that 100% satellite coverage will be available across a latitude range between 20° and 40°. However, if the minimum elevation angle is increased to 20°, then the percent of coverage time will be decreased as shown. Nevertheless, as the satellite-based telecommunications system grows from the original constellation to the full baseline constellation, it should be noted that the percent of continuous coverage time will increase over a considerably wider latitude band. For example, in the full baseline constellation of Figure 8, continuous 24-hour visibility will be provided in a latitude range between 10° and 90° for a minimum elevation angle of 20°. However, again it should be noted that the land mass coverage provided in these latitude ranges is achieved in connection with the coordinated boresight steering method according to the present invention. While the FOV for the satellite antennas could be enlarged so that NADIR pointing satellites with a 42° FOV are used, this would not be as advantageous because a considerably higher antenna gain would be necessary, and the size of the antennas would be increased as well. Even though a 42° FOV is less than the 44° FOV of the satellite (full width of the earth), fixed NADIR pointing for the boresight would be acceptable for global land mass coverage. Figure 11 shows another illustration of satellite visibility for the original constellation of Figure 7. However, in this figure, the time gaps are graphically illustrated and quantified. Referring now to Figures 12A-12E, a cartographic illustration of the "coordinated" boresight steering method according to the present invention is shown. Each of these figures show the movement and steering of the six satellites employed in the original constellation of Figure 6 over a period of approximately one hour. For purposes of simplicity, these six satellites are labeled with reference numerals 1 through 6. This sequence of figures is based upon the satellite antennas (e.g., antennas 16 and 18) having a FOV of 40°. A shaded area is provided for each of these satellites, such as delineated area 48 for satellite 5. These delineated areas represent the 40° FOV for the satellite antennas. Each of these areas include three arcuate lines, such as lines 50-54 for satellite 5. The outer line 50 represents a minimum elevation angle of 10°, the center line 52 represents an elevation angle of 20° and the inner line 54 represents an elevation angle of 30°. Each of the satellites 1-6 in Figures 12A-12E also include two associated lines, such as lines 56 and 58 for satellite 5. Line 56 represents the direction of orbital travel for satellite 5, while line 58 is representative of the boresight line of sight for satellite 5 to an aim point on the ground. In other words, line 58 shows that the boresight of satellite 5 is pointed at an angle from NADIR which is greater than zero. In this case, the antennas 16-18 of satellite 5 are pointed toward North America. With a 40° FOV for the satellite antennas, it is anticipated that the maximum swing in boresight angle relative to NADIR will be approximately ±4°. This is in contrast to the wider boresight steering which is employed in the embodiments of Figures 1 and 2, where the anticipated that the maximum swing in boresight angle relative to NADIR will be approximately ±20°. This difference in boresight swing is due to smaller antenna FOV preferred for the embodiments of Figures 1 and 2 (e.g., 23°), and the use of a fixed aim point location in those embodiments. As shown in Figure 12A, satellite 5 covers almost all of the United States at a minimum elevation angle of 10°, while satellite 1 covers all but the most northern countries of Europe at this elevation angle. With respect to the United States, satellite 6 is available to cover the most north eastern of its states. Additionally, Figure 12A shows the provision of an overlap interval between satellites 5 and 6 over the United States. In this case, satellite 5 is the arriving satellite, while satellite 6 is the departing satellite. Accordingly, as indicated above, satellite 5 will be used to handle all newly initiated telephone calls in the overlap interval with satellite 6. Figure 12B illustrates the satellite coverage provided by the original constellation at a time period which is fifteen minutes later than that shown for Figure 12A. The remaining set of figures in the Figure 12A-12E sequence also illustrate the original satellite constellation at consecutive fifteen minute intervals. In light of the approximately six hour orbital period for the satellites 1-6, these satellites will move around the earth at a generally fixed rate of travel. As a result, it should be appreciated that the coordinated boresight steering method according to the present invention will minimize, if not eliminate, any coverage gaps over land above the minimum elevation angle of 10°. The boresight steering method is referred to as being coordinated in that the adjustment of the boresight angle for one satellite will at many times require a change in the boresight angle of an adjacent satellite. Thus, for example, a change in the boresight angle of satellite 2 may require a change in the boresight angle for both satellites 1 and 3. The method of boresight steering is dramatically illustrated in the sequence of Figures 13A-13G. In this case, a 35° FOV for the satellite antennas is used to more fully illustrate the concept relative to a single satellite 60. More specifically, Figures 13A-13G show the beam pattern of satellite 60 as it travels over Europe during a time period of two hours. Each of the circles, such as circles 62 and 64, represent the coverage provided by one beam of a multibeam antenna, such as antenna 16 of Figure 1. Line 66 represents the minimum elevation angle of 10°. Line 68 represents the boresight line of sight to the ground. Accordingly, in Figure 13A, the boresight points forward as the satellite 60 approaches Europe, and the boresight angle relative to NADIR decreases in Figures 13B-13C, until the boresight aim point is approximately at NADIR in Figure 13D. Then, as the satellite continues its travel, the boresight is steered to a location looking behind the satellite in Figures 13E-13G. It should also be noted that each of the beams are labeled in Figure 13D to illustrate that the antenna includes an array of 35 beams. In accordance with the present invention, the physical boresight steering may be accomplished through a variety of mechanisms, such as adjusting the attitude of the satellite itself ("body steering"), the use of gimballed antennas, or possibly even through electronic focusing (e.g., with a phase array antenna). However, in accordance with the coordinated boresight steering method of the present invention, each of the satellites 1-6 (or satellites 40a-40b, 42a-42b and 44a-44b of Figure 6) would be programmed from a terrestrial-based station with the command signals or other data necessary to control the boresight steering mechanism for a predetermined orbital period. This predetermined orbital period should comprise a significant orbital duration, which will avoid unnecessary ground to satellite transmissions. Thus, for example, the predetermined period may be the repeating 24-hour orbital period which occurs with a satellite altitude of 10360 km (5600 nautical miles). As an example of one coordinated boresight steering method according to the present invention, computer generated plots such as those shown in Figures 12A-12E are used to determine the most appropriate boresight locations of all of the satellites at various intervals during the predetermined orbital period. While these boresight aim points may be determined manually, it should be appreciated that this process could be automated with an appropriate rule set. In this regard, the Antenna Beam Aim Table ("ABATB") below set forth the boresight locations for each of the six satellites of the original constellation over a six hour period. The first portion of the table identifies the configuration of the satellite constellation. More specifically, a 6/3/3 phasing relationship is used in this example, meaning that there are six satellites in three orbital planes with a 3 x 60 degree phasing relationship between satellites in different orbital planes. The column labeled Time indicates a point in minutes from the starting time reference. The second column shows the Latitude at this given time, while the third column shows the Longitude at this given time. Thus, for example, at Time = 120 minutes, the antennas 16-18 of the first satellite will be pointed at Latitude 43.8° and Longitude 100.2°, while the antennas of the third satellite will be pointed at Latitude 44.4° and Longitude -121.2°. In contrast, it will be noted that there is no time entry in the table for the second satellite at Time = 120 minutes. Rather, the closest time entries for the second satellite are for Time = 90 minutes and Time = 180 minutes, respectively. This is not to say that the boresight of the second satellite remains fixed during this 90 minute interval between Time = 90 minutes and Time = 180 minutes. In accordance with one method step under the present invention, the boresight location may be interpolated between each time entry in the table, such as through straight line approximation between two points (as indicated by the last column of the table). The difference between each of the time entries in the table is related to the speed at which the boresight angle needs to be adjusted. Thus, for example, in several instances of the table, the time difference may only be 10 minutes when rapid movement is necessary to maintain continuous 24-hour coverage. In other words, it should be understood that the antenna boresight of the satellites will be steered at a variable rate in accordance with the present invention. For example, when a satellite is arriving or leaving a particular region of the earth, such as Europe, the rate at which the boresight angle is changed will be greater than the rate of change required when the satellite is more directly over its current region of coverage. In any event, it should be understood that the present invention may be utilized with a variety of different satellite attitude control systems. Thus, while boresight locations over time could be uplinked to each of the satellites, it should also be understood that other command information could be supplied as well, such as steering rate commands relative to time. Referring to Figures 14-16, these figures will be used to illustrate an alternative original constellation embodiment. The principal distinction between this original constellation embodiment and that of Figure 6 is that each of the six satellites 40a-40b, 42a-42b and 44a-44b will reside in their own individual orbital planes, each of which are inclined at a preferably 53° angle relative to the equator (e.g., plane P 7 ). Accordingly, the preferred phasing for this alternative embodiment will be 6/6/4. Figures 14-16 generally correspond to Figures 9-11 in order to more fully illustrate and contrast this embodiment with the previous original constellation embodiment. In this regard, Figure 14 shows that the band of continuous satellite visibility has affectively shifted upwardly, so that now all but the most southern portion of Europe is visible. In other words, the original constellation of Figures 6 and 9-11 is directed towards initial coverage of the United States, while the alternative constellation of Figures 14-16 is directed towards initial coverage of Europe. Nevertheless, Figure 16 demonstrates that only two relatively minor gaps in coverage would result for telecommunications with users located in the United States. In either case, the preferred altitude of the satellite constellation is still 10360 km (5600 nautical miles). Finally, Figure 17 provides a flow chart which illustrates one technique for implementing the coordinated boresight steering method according to the present invention. The first step will be to initialize the satellite configuration (block 70), as indicated in the Antenna Beam Aim Table discussed above. The boresight for each of the satellites may be initially set to a predetermined orientation, such as NADIR. The next step will be to determine the most appropriate boresights for each of the satellites at Time = 0 (block 72). A chart such as that shown in Figure 12A could be used to guide this determination. However, it should be understood that this process may be further automated with appropriate programming. Once the boresights at Time = 0 are determined, then the time period should be incremented by a predeterminable amount to consider whether a change in the boresight steering rate is needed for the first satellite (block 74). For example, a minimum time period of five minutes could be used. However, it should be appreciated that other suitable time periods could be selected, depending in part upon the altitude of the satellites and the speed at which they are orbiting the earth. Accordingly, at Time = 5 minutes a decision should be made as to whether the boresight steering rate should be changed for the first satellite (diamond 76). In the event that a steering rate change is required, then a new boresight aim point will be determined (block 78). This new boresight aim point may be stored as an entry in the Antenna Beam Aim Table, as discussed above. If a rate change is not required in order to maintain the desired coverage, then this determination should be made for each of the other satellites in turn before another time increment is considered. Accordingly, Figure 17 illustrates a check to see if the current satellite being considered is the last satellite in the constellation (diamond 80). If the current satellite is not the last satellite, then the next satellite in the constellation will be examined (block 82). Once all of the satellites in the constellation have been examined for coverage at the current time increment, a check will be made to see if this time increment is the last time increment in the predetermined time period over which rate adjustments will be considered (diamond 84). If the current time increment is not the last time period, then the time period will be incremented and the process will be repeated. Once the complete time or orbital period has been evaluated, then an interpolate step may be performed to convert the sequence of boresight aim points in the table to rate commands (block 86). Finally, these rate commands or other suitable data for adjusting the boresight will be transmitted to each of the satellites in the constellation (block 88). These rate commands may be transmitted from one or more ground-based control stations, such as gateway station 12 shown in Figure 1. For example, the rate commands could be uplinked from one ground-based control station as each satellite passes in turn over the region of coverage in which the ground-based control station is located. However, in the event that not all of the satellites in the constellation cover the region where the ground-based station is located, then an additional ground-based station may be employed to transmit the rate commands to pre-selected satellites. From the foregoing, it will be appreciated that the present invention represents a significant advance in the field of satellite-based cellular telecommunications systems. Although several preferred embodiments of the invention have been shown and described, it will be apparent that other adaptations and modifications can be made without departing from the scope of the invention. Accordingly, the invention is not to be limited, except as by the following claims.

Claims: A method of controlling the communication between at least one mobile cellular telephone (14), one of a plurality of telecommunication satellites (10) forming a satellite constellation and at least one other mobile cellular telephone (14) or a gateway station (12) in a cellular telephone communication system, comprising the steps of: determining a sequence of different focal directions over a predetermined orbital period for at least one antenna (16, 18, 20, 22) on each of said satellites (10), with the focal direction of said at least one antenna (16, 18, 20, 22) on each of said satellites (10) being adjusted at a rate which is varied during said predetermined orbital period, and periodically transmitting command information comprising data to each of said satellites (10) from at least one earth-based control station suitable for adjusting the focal direction over said predetermined orbital period of said at least one antenna (16, 18, 20, 22) on each of said satellites (10) in accordance with said sequence, wherein the determination of the sequence of different focal directions of said at least one antenna (16, 18, 20, 22) for each of said satellites (10) is performed in a coordinated manner taking into account the sequence of focal direction changes of at least one antenna (16, 18, 20, 22) of an adjacent satellite (10) during at least a portion of said orbital period, wherein the satellite constellation provides continuous 24-hour satellite visibility only to restricted regions of the earth, and wherein the number of satellites, the field of view of said satellites and the orbital configuration of said satellite constellation combine to provide 24-hour cellular telephone communication coverage over a predetermined range around the world which is less than full global coverage. The method according to claim 1, wherein said command information that is transmitted to each of said satellites (10) comprises boresight locations over time as well as varying steering rate commands relative to time. The method according to claim 1, wherein the rate at which the focal direction is adjusted is greater when a satellite is arriving or leaving a particular region of coverage, than the rate of adjustment required when the satellite is more directly over its current region of coverage. The method according to claim 1, wherein the boresight of the satellite's antennas remains centered on a given assigned service region while said service region is visible to said satellite. The method according to claim 1, wherein each of said satellites (10) achieve said sequence of focal directions by electronic focusing. The method according to claim 1, wherein each of said satellites (10) achieve said sequence of focal directions by changing the attitude of the satellite (10). The method according to claim 1, wherein each of said satellites (10) achieve said sequence of focal directions by changing the attitude of said antennas (16, 18, 20, 22) relative to the attitude of the satellite (10). The method according to claim 1, wherein said sequence of focal directions is determined by selecting a particular focal direction at individual time intervals during said predetermined orbital period, and interpolating a plurality of focal directions in between each of said selected focal directions. The method according to claim 8, wherein the duration of said individual time intervals is related to the speed at which the boresight angle needs to be adjusted. The method according to claim 1, wherein said command signals are transmitted to each of said satellites (10) at times which are related to the orbital periods of said satellites (10). The method according to claim 1, wherein said command information includes the information needed by said satellites (10) to implement said sequence of focal directions over one 24-hour period.

Inventors: CRESS PETER H, HORSTEIN MICHAEL, RUSCH ROGER J, CRESS, PETER H., HORSTEIN, MICHAEL, RUSCH, ROGER J.
Applicants: TRW INC, TRW INC.
Application year: 1994
Main classification: H04B7
IPC codes: B64G1, H04Q7, H04B7